What's more, small child stifles exclamation of disapproval (2,4)
I believe the answer is:
to boot
'what's more' is the definition.
(I've seen this before)
'small child stifles exclamation of disapproval' is the wordplay.
'small child' becomes 'tot' (I've seen this before).
'stifles' means one lot of letters goes inside another.
'exclamation of disapproval' becomes 'boo' (I've seen this before).
'tot' enclosing 'boo' is 'TO BOOT'.
(Other definitions for to boot that I've seen before include "Kick" , "on top of everything else" , "In addition - using heavy footwear" , "Where an extra case may be added" , "What's more" .)